Method and apparatus for minimizing propagation losses in wavelength selective filters

ABSTRACT

The present invention is a method and an apparatus for minimizing losses in wavelength selective filters. In one embodiment, an apparatus includes a waveguide bus defined in a first crystalline layer of the apparatus, for receiving incoming light, a resonator defined in the first crystalline layer, and a coupling structure defined in a second polysilicon or amorphous silicon layer of the apparatus, for coupling a selected wavelength of the incoming light from the waveguide bus to the resonator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/777,023, filed Jul. 12, 2007 now U.S. Pat. No. 7,469,085, (currentlyallowed) entitled “METHOD AND APPARATUS FOR MINIMIZING PROPAGATIONLOSSES IN WAVELENGTH SELECTIVE FILTERS”, which is herein incorporated byreference in its entirety.

BACKGROUND

The invention relates generally to optics, and relates more particularlyto optical interconnects.

FIG. 1A illustrates a top view of one example of a conventionalwavelength selective filter 100 (e.g., such as those used for wavelengthdivision multiplexing). FIG. 1B illustrates a cross sectional view ofthe filter 100 of FIG. 1A, taken along line A-A′. The filter 100includes a ring resonator 102 side-coupled to an access straightwaveguide (or waveguide bus) 104. The ring resonator 102 is tuned to awavelength channel of interest, such that the ring resonator 102 filtersthis channel from the bus 104. For high refractive index contrast planarlightwave waveguides and circuits, the coupling gap 106 (i.e., thedistance that separates the ring resonator 102 from the bus 104) istypically on the order of a micrometer and controlled within a fewnanometers of precision. Such control, however, is difficult to achieveby typical lithography methods.

FIG. 2A illustrates a top view of an alternative example of aconventional wavelength selective filter 200. FIG. 2B illustrates across sectional view of the filter 200 of FIG. 2A, taken along lineA-A′. Like the filter 100, the filter 200 includes a ring resonator 202coupled to a waveguide bus 204. However, as illustrated in FIG. 2B, thering resonator 202 is formed in a high refractive index waveguidinglayer that is separate from the layer in which the bus 204 is formed. Inthis case, the coupling gap 206 is vertically disposed and can beprecisely controlled by an amount of gap material grown, for example, bymolecular-beam epitaxy (MBE).

For silicon on insulator (SOI)-based planar lightwave circuits based onstrip silicon single-mode waveguides with sub-micron cross sections, theapproach illustrated in FIGS. 1A and 1B results in a coupling gap on theorder of 100 nanometers, which should be controlled with nanometerprecision. This makes fabrication tolerances very difficult to maintain.Applying the alternative approach illustrated in FIGS. 2A and 2B wouldrequire the growth of an oxide or other low refractive index material ontop of the SOI structure to form the coupling gap, followed by growth ofan additional top silicon layer for the ring resonator. This is likelyto result in a polycrystalline or amorphous silicon structure on top ofthe silicon layer, which can lead to significant propagation losses(e.g., approximately twenty dB/cm) due to scattering on the grainboundaries. Losses are increased proportionally to the photon lifetime(inverse of the ring resonator quality factor) if the ring resonator orother resonator structure is located on the top layer of the circuit.

Thus, there is a need for a method and an apparatus for minimizingpropagation losses in wavelength selective filters.

SUMMARY OF THE INVENTION

The present invention is a method and an apparatus for minimizing lossesin wavelength selective filters. In one embodiment, an apparatusincludes a waveguide bus defined in a first crystalline layer of theapparatus, for receiving incoming light, a resonator defined in thefirst crystalline layer, and a coupling structure defined in a secondpolysilicon or amorphous silicon layer of the apparatus, for coupling aselected wavelength of the incoming light from the waveguide bus to theresonator.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe obtained by reference to the embodiments thereof which areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1A illustrates a top view of one example of a conventionalwavelength selective filter;

FIG. 1B illustrates a cross sectional view of the filter of FIG. 1A,taken along line A-A′;

FIG. 2A illustrates a top view of an alternative example of aconventional wavelength selective filter;

FIG. 2B illustrates a cross sectional view of the filter of FIG. 2A,taken along line A-A′;

FIG. 3A is a top view of one embodiment of a wavelength selectivefilter, according to the present invention;

FIG. 3B is a cross sectional view of the filter of FIG. 3A, taken alongline A-A′; and

FIG. 3C is a cross sectional view of the filter of FIG. 3A taken alongline B-B′.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

In one embodiment, the present invention is a method and an apparatusfor minimizing propagation losses in wavelength selective filters.Embodiments of the present invention vertically couple a waveguide busto a resonator using a straight polysilicon waveguide section withlateral tapers, where the waveguide bus and the resonator are located ona common crystalline layer of an SOI wafer.

FIG. 3A is a top view of one embodiment of a wavelength selective filter300, according to the present invention. FIG. 3B is a cross sectionalview of the filter 300 of FIG. 3A, taken along line A-A′. FIG. 3C is across sectional view of the filter 300 of FIG. 3A taken along line B-B′.Referring simultaneously to FIGS. 3A-3C, the filter 300 comprises aresonator (e.g., a ring resonator) 302, a waveguide bus 304 and acoupling structure 308.

The resonator 302 and the waveguide bus 304 are defined in a first,common crystalline layer 312 of the SOI wafer of the filter 300. Thecoupling structure 308 is defined in a second layer 314 of the filter300, located in one embodiment above the first layer 312. In oneembodiment, the second layer comprises polysilicon or amorphous silicon.The coupling structure 308 comprises a substantially straight waveguidehaving lateral adiabatic tapers 310 ₁-310 ₂ (hereinafter collectivelyreferred to as “tapers 310”) at each end. The tapers 310 are configuredfor coupling incoming light between the coupling structure 308 and thewaveguide bus 304 or the resonator 302.

Specifically, incoming light is received in the first layer 312 of thefilter 300 by a first section of the waveguide bus 304. As the lightpropagates through the waveguide bus 304, it is coupled to the couplingstructure 308 in the second layer 314 of the filter 300, via a firsttaper 310 ₁. The light then propagates through the coupling structure308 until the light reaches the resonator 302, where the selectedwavelength is coupled to the resonator 302. The remainder of the light(i.e., wavelengths other than the selected wavelength) continues topropagate along the coupling structure 308 until the light reaches asecond taper 310 ₂, by which the remainder of the light is coupled backto the waveguide bus 304.

The filter 300 is thereby configured to control the coupling gap 306between the resonator 302 and the waveguide bus 304 by growth (e.g., ofoxide or other low refractive index material). However, propagationlosses in this case are minimized because the optical mode of incominglight propagates in the polysilicon or amorphous silicon top layer(i.e., second layer 314) for only a very short relative distance, and nofurther substantial losses due to the resonator 302 occur.

Thus, the present invention represents a significant advancement in thefield of optics. Embodiments of the present invention provide a couplingsection (e.g., a straight poly-silicon waveguide section with lateraltapers) by which a waveguide bus is vertically coupled to a resonator.The gap between the bus and the resonator can be tightly controlled,while propagation losses are minimized.

While foregoing is directed to the preferred embodiment of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An apparatus, comprising: a waveguide bus defined in a firstcrystalline layer of the apparatus, for receiving incoming light; aresonator defined in the first crystalline layer; and a couplingstructure defined in a second layer of the apparatus, for coupling aselected wavelength of the incoming light from the waveguide bus to theresonator, wherein the coupling structure comprises: a waveguide havingat least one lateral, adiabatic taper for coupling the incoming lightbetween the coupling structure and the waveguide bus or between thecoupling structure and the resonator.
 2. An apparatus, comprising: awaveguide bus defined in a first crystalline layer of the apparatus, forreceiving incoming light; a resonator defined in the first crystallinelayer; and a coupling structure defined in a second layer of theapparatus, for coupling a selected wavelength of the incoming light fromthe waveguide bus to the resonator, wherein a coupling gap is formedbetween the first crystalline layer and the second layer, the couplinggap being controllable by oxide growth.